The present invention relates to a circuit and a method for monitoring and diagnosing an oxygen probe.
As is known, internal combustion engines are provided with systems for exhaust gas reduction, based on the information provided by oxygen probes (the so-called LAMBDA probes), and electronic control units whose tasks include, among other things, minimising emissions of pollutant substances and verifying, with the assistance of appropriate circuits, the correct operation of the various components, in particular the oxygen probes. These probes normally comprise a sensitive cell having an internal resistance and providing a voltage signal whose amplitude varies as a function of the oxygen concentration present in the atmosphere in which the sensitive cell is immersed, and a heating member, for instance formed by a resistor, which makes it possible to maintain the temperature of the sensitive cell within a predetermined range of operating values, for instance between approximately 600xc2x0 C. and approximately 800xc2x0 C.
Various types of circuits and relative methods for monitoring and diagnosing oxygen probes are known.
According to a first solution, a resistor having a known resistance value is disposed in series with the heating member, while a differential amplifier detects the voltage drop at the resistor and supplies a control unit of the electronic control unit with an output signal representative of this voltage drop. According to the solution described above, the output signal of the differential amplifier is digitised and is used to calculate, in a manner known per se, the current flowing in the resistor and in the heating member and to regulate, for instance by means of a controller of proportional-integral type, the power dissipated by the latter. The control unit, verifies, moreover, whether the current is maintained within a predetermined range of operating values. If not, a malfunction of the heating member is detected.
A second solution entails the use of a differential amplifier having inputs connected to the terminals of the sensitive cells and an output connected to the control unit. In this case, a current is injected into the probe in order to estimate its internal resistance which is linked to the temperature of the probe according to a known relationship. The temperature of the probe may be adjusted again using a controller of proportional-integral type.
Although the solutions described above are accurate and reliable, they nevertheless entail drawbacks as they make use of costly circuits, in particular as a result of the differential amplifier and, in the first case, also of the shunt resistor.
The object of the present invention is to provide a circuit for monitoring and diagnosing an oxygen probe which is simple and economic to construct.
The present invention therefore relates to a circuit for monitoring and diagnosing an oxygen probe, provided with a sensitive cell, connected between a first and a second signal terminal, and a heating member, connected between a battery and a heating terminal, the circuit comprising an interface stage and a control unit, this interface stage comprising a first resistor connected between a supply line and the first signal terminal of the oxygen probe, a second resistor connected between the first signal terminal and the second signal terminal of the oxygen probe, the circuit being characterised in that the control unit comprises first analog-digital converter means connected to the first signal terminal, and a second analog-digital converter means connected to the second signal terminal, and in that the interface stage comprises a third resistor connected between the second signal terminal and a reference potential line.
The present invention also relates to a method for monitoring and diagnosing an oxygen probe, as defined in claim 5.